The steam heating of a paper sheet is widely practiced in papermaking. The increase in sheet temperature that results provides increased drainage rates for the water thus reducing the amount of water to be evaporated in the drier section. Water drainage is improved by the application of steam principally because the heating of the sheet reduces the viscosity of the water, thus increasing the ability of the water to flow. Most of the heat transfer takes place when the steam condenses in the sheet. The condensation of the steam transforms the latent heat of the steam to sensible heat in the water contained by the sheet.
A particular advantage of the steam heating of the paper sheet is that the amount of steam applied may be varied across the width of the sheet along the cross machine direction so that the cross machine moisture profile of the sheet may be modified. This is usually carried out to ensure that the moisture profile at the reel is uniform. Techniques in the papermaking art for sensing the moisture profile of a sheet of paper are well known. If a sensing apparatus is positioned over the paper sheet, downstream of a steam distributor able to control the moisture profile, then after measuring the water profile in the sheet, steam can be applied in varying amounts on a selective basis across the sheet, thus achieving the required uniform moisture profile at the reel.
It is known to divide a steam distributor into compartments and to control the supply of steam to each compartment, thus controlling the moisture profile of the sheet. Fiber and dirt accumulate within the compartments and over time, the debris penetrates into the internal structures and interferes with steam flow. The steam distributor must be disassembled in order to clean the internal components.
U.S. Pat. No. 5,711,087 to Pazdera describes an apparatus for distributing steam to a paper web or calendar roll which includes a removable curved-shaped profile screen. The screen is mounted on the apparatus with clip members that interrupt the otherwise smooth exterior surface of the screen. In addition, the use of external clip members makes the removable screen susceptible to flexing outward with increasing steam pressure. Moreover, the clamped edge of the screens must often be separated from the clips on the frame using jarring force, then pried back into place. When they are reattached, the screens lose the intended tight fit against the baffles thereby allowing significant leakage between profiling zones. Finally, in these prior art designs where the screens are not permanently attached, the steam holes in the screen must be situated near either the leading or trailing edge of the steambox in order to minimize the machine direction (MD) length of the screen. Consequently, if a screen becomes too long in the MD, the screen tends to bow out which causes excessive and inconsistent leakage between profiling zones. These removable screen plates become warped and battered after only a few cleaning routines.
U.S. Patent Application 2006/0107704 to Passiniemi describes a steam distribution apparatus that is partitioned into a number of discharge chambers and includes screen plates which are welded to the partitions to prevent the screen plates from twisting or flexing. While the apparatus includes sealable slots for access to the internal compartments for cleaning, the slots afford only limited access.